Within the framework of the present invention, low-altitude flight is understood to mean flight along a flight trajectory (at low altitude) allowing an aircraft to follow as closely as possible the terrain overflown, especially so as to avoid being pinpointed. Such a flight trajectory at low altitude is therefore situated at a predetermined terrain height, for example 500 feet (about 150 meters).
Although not exclusively, the present invention applies more particularly to a military transport plane which exhibits a low thrust/weight ratio and a high inertia, and whose maneuvering times are in general relatively slow. In the event of detection of a ground-air threat (visually or else by an onboard electronic counter-measures system), such a military transport plane must be capable, for obvious safety reasons, of rapidly performing a course modification, which modification was not programmed at the outset.
However, the determination of a low-altitude flight trajectory requires a very high calculational power on the part of the aircraft's onboard systems. As a course modification that makes it possible to exit a hostile zone as soon as possible must be able to be performed at almost no notice, the pilot cannot manually reschedule a new course. Specifically, firstly, such a manipulation requires time (point by point rescheduling), and secondly the calculation of the low-altitude trajectory over a new lateral course also requires a certain delay. These various times are of the order of several seconds, and this may be much too long in certain situations to enable the aircraft to distance itself from a hostile zone, in particular during an aforesaid threat of ground-air type.